This invention relates to the production of polyolefins in fluidized beds, and particularly to an efficient method of removing polymerized product from a fluidized bed reactor at a desired rate of production while optimizing the conservation of fluidizing gas.
In the production of poloyolefins in fluidized bed reactors, a long standing problem has been the loss of unreacted monomers and other components of the fluidizing gas as a consequence of removing the solid product from the reactor. The polymer product is in the form of small particles and is generally removed by passing them, usually by positive gas pressure transfer assisted by gravity, while still more or less entrained in, or at least surrounded by, the gaseous atmosphere predominating in the reactor. Regardless of the system of valves which causes the flow of solid product to the desired destination, the gas leaving the reactor along with the product generally contains a significant volume of unreacted monomer. This unreacted monomer represents an economic loss in at least two waysxe2x80x94it is no longer in the reactor where it can form the desired product, and it represents an environmental and safety issue which must be dealt with. The fluidizing gas may include inert gases used, for example, to control the dew point of the recycle gas. This also represents an economic loss. It is therefore desirable to minimize the amount of gas accompanying the product as it leaves the reactor. There is also a secondary effect, arising from the care taken to minimize the removal of gas with the finished productxe2x80x94the systems developed to assure only a minimal loss of gas tend to retard the removal of solid product, which can cause the entire process to be dependent on the rate of product removal rather than the otherwise possible rate of production. It is highly undesirable for the production rate of a large, expensive reactor to be limited by the product removal system.
In Aronson""s U.S. Pat. No. 4,621,952, it is pointed out that the original Union Carbide fluidized bed process for making polyolefins included a gas lock zone associated with the product discharge train. Unreacted monomer accompanying the product resin was vented and recycled back to the reactor by compression. See U.S. Pat. Nos. 4,003,712, 4,032,391, 4,255,542, and 4,302,565. The Aronson ""952 patent describes the use of a settling zone to fill a vessel with as much solid material as is practical, thus minimizing the amount of gas in the settling zone or vessel before it is sealed off by appropriate valving from the rest of the system. It also describes a transfer zone downstream from the settling zone. While discharging to the transfer zone, the settling zone reaches an intermediate pressure which is then preserved and can be increased after the settling zone is emptied by connection to another settling zone containing higher pressure gas, to reduce the amount of gas that can enter the settling zone from the reactor in the next cycle.
In U.S. Pat. No. 4,535,134, the loss of gaseous monomer during solid product removal from a horizontal reactor is reduced by controlling the powder level in a receiving container.
Our invention is a method of product removal from a fluidized bed polyolefin reactor which optimizes the conservation of monomer and other gas components within the time available for the product discharge function. The invention calls for a plurality (preferably two) parallel sets or series of vessels for receiving product from the reactor. The product is passed, in each parallel train, first to a first stage vessel, then to a second stage vessel having a reduced pressure, and finally to a conveyor, bin, or other end destination of a pressure lower than that of the second stage vessel. The conveyor, bin, or other end destination may have a pressure lower than atmosphericxe2x80x94for example, where gas is drawn from it by a vacuumxe2x80x94but may also be atmospheric or higher than atmospheric. Optionally, a third stage vessel of further reduced pressure may be inserted between the second stage vessel and the final destination. After each discharge from one vessel to another, connection is made between the just-filled vessel and the equivalent one in the other train, so that gas may pass from the higher pressure vessel to the lower pressure vessel. Appropriate valves are manipulated by a control system which takes into account the pressures and volumes of gas in both vessels over time, the effect (optionally) of solids content in the vessel on the volume of gas in the vessel, the flow resistance characteristics of the connections between the vessels, and the overall product discharge cycle time. On-line determination of the flow and fill characteristics of the current product can be utilized with or without historical data. The control system allocates a time for equalization at each stage to permit the most efficient sequence of stepsxe2x80x94that is, to permit the removal of all product made within the discharge cycle time period, with a minimum or optimum level of gas loss. The sequence of steps includes as an option the simultaneous performance of certain valve operations. That is, more than one step may be performed at the same time, as will be seen below.